The present invention relates to the protection of pitot static tubes and, in particular, an improved wear and heat resistant probe cover installable over pitot static probes of aircraft.
Pitot static tubes are conventionally used on aircraft for measuring speed and altitude. Because of sensitivity and delicacy, such instruments are prone to damage during installation, maintenance, and storage. To avoid such contamination, for any appreciable layover, the pitot static tubes are covered temporarily to prevent dust, particulates and other foreign matter from entering the probe ports. Prior to flight the probe covers are removed.
Many aircraft, primarily commercial, also employ heating devices on the pitot static probes to prevent icing at the probe ports that can adversely effect the accuracy thereof. Failure to remove the covers prior to heating, in addition to disabling the probe, can cause damage to the probe resulting from cover material degradation.
Damage from such inadvertent failure to remove the cover has resulted in the use of fire resistant materials for such protection. As disclosed in U.S. Pat. No. 5,127,265 to Williamson et al., the body of the pitot static tube cover may be a braided material comprised of fiberglass that is pretreated to remove sizing and organic residue. Such design is currently in commercial usage and does provide protection against thermal degradation in the presence of inadvertent thermal cycles.
Nonetheless, problems of probe contamination can occur with the fiberglass construction during normal and customary usage. Inasmuch as the tubes are made of braided material, they are longitudinally flexible and crimp during insertion and removal. Accordingly, the cover interior tends to abrade and erode in use resulting in fine glass particulates that collect on the interior surfaces of the cover. The gathered, closed tail end of the cover also presents exposed frayed strands that can abrade over time presenting an additional source of particulates. Furthermore, the mouth end of the cover is very difficult to visually align inasmuch as the pitot-static tubes on commercial aircraft may be a considerable distance above ground. Accordingly, the mouth generally impacts the sharp frontal edge of the ram port and significantly degrades with use resulting in yet another source of probe contamination. All such particulates may enter the probe orifices and effect operation and accuracy of the monitoring systems.
In order to overcome such potential problems, installation and removal procedures have been developed to limit abrasive contact between the probe and the cover material. Additionally, the covers are provided with horizontally opposed lifting tabs for use in conjunction with positioning equipment to facilitate installation on larger aircraft wherein direct manual control of the placement is not possible. However the cover is prone to sagging in such prior art covers wherein the stitched seam on the closed end is also horizontally disposed. During installation, even with the above lifting tools, the drooping braided material is abraded by the probe port. In actual practice, however, total compliance and proper placement is not achieved and all parts of the covers degrade over time for the above reasons presenting ongoing problems in assuring that prior to flight probe contamination has not occurred. Still further, the spring at the mouth end of the probe cover has been formed by hooking together the ends of a coiled spring. Such an arrangement produces inherently a non-circular, tear drop shape that is transferred to the assembled cover. This non-circularity further increases the abrasive contact generating a further source of internal particulates that can clog the probe ports. Furthermore, the closed or tail end of the cover is formed by inverting a free end into the cover body and sewing the fabric at a seam. Accordingly, the free end fibers readily unwind and fracture in use generating a further source of particulates. Thus a need continues for a pitot static tube cover that will be thermally resistant, and easily installed and removed using accepted procedures and equipment while withstanding the limitations of periodic irregularities and non-compliance on installation and removal procedures.
Additionally, the above-mentioned glass filament requires time consuming and expensive preprocessing to prepare the braided tube for manufacture. Such preprocessing requires heating the fabric to an elevated temperature to remove residuals and contaminants as well as annealing the fabric to permit cutting and shaping without excessive fragmentation. Thus a further need exists for a pitot static tube cover material resistant to the temperature transients presented by inadvertent heater energization, formable without heat treating, resistant to abrasive filament degradation, and useable in accordance with normal practices without generating particulate contamination.
The present invention provides an improved pitot static probe cover overcoming the material limitations of the foregoing and withstanding the realities of normal usage. The probe cover is in the form of a tubular braided material selected from woven fibers possessing high thermal stability, strength and abrasion resistance. Para-aramid continuous filaments are particularly well suited for such covers and do not require preprocessing or annealing. Such materials provide substantial thermal protects, have high tensile and abrasive strength, and are readily sized and configured to the shapes required for a variety of probe designs. The problem of impact fragmentation, for both the glass material and the para-aramid materials, is reduced by encapsulating the mouth end of the probe with elastomeric saturant that resists abrasion during misaligned installation. Fragmentation at the closed tail end of the tube due to fraying of the fabric end is reduced by enclosing the ends in a rearward pocket and orienting the stitched seams transverse to the probe lifting loops. The problem of non-circularity caused by the prior art biasing spring are overcome by incorporating a limiting ring inside the spring coils that prescribes circularity and initial biasing while limiting diametrical expansion. In another embodiment, controlled biasing at the mouth interface is provided by wrapped filaments encapsulated in a silicone rubber thereby providing a biasing that does not abate over the service life and is particularly useful in probe designs wherein probe body taper can not be used for retention compression.
Accordingly, it is an object of the present invention to provide a pitot probe cover resistant to abrasive wear while resisting thermal degradation in the presence of applied heat from the probe heaters.
Another object of the invention is to provide a flexible pitot probe cover retaining structural integrity under normal servicing procedures.
A further object of the invention is to provide pitot tube cover having controlled compressive fitting with the pitot tube body for extended periods.
Yet another object of the invention is to provide a cover for aircraft pitot static tube of a high strength braided material not requiring preconditioning and not subject to abrasive wear in usage.